1. Field of the Invention
This invention relates to a displacement type fluid machine such as a compressor, a pump and an expander.
2. Description of the Related Art
As displacement type fluid machines, there have been long known a reciprocating fluid machine in which a piston is reciprocally moved repeatedly in a cylinder to move a working fluid, a rotary (rolling piston-type) fluid machine in which a cylindrical piston makes an eccentric rotary motion in a cylinder to move a working fluid, and a scroll fluid machine in which a pair of stationary and orbiting scrolls, each having a wrap of a volute configuration formed perpendicularly on an end plate, are engaged with each other, and a working fluid is moved by revolving the orbiting scroll.
The reciprocating fluid machine has an advantage that it can be easily manufactured, and is inexpensive since its construction is simple, but a stroke from the end of the suction to the end of the discharge is as short as 180.degree. in terms of an angle of rotation of a shaft, and the flow velocity during the discharge stroke becomes high, which invites a problem that the performance is lowered because of an increased pressure loss, and since the motion for reciprocating the piston is required, the rotation shaft system can not be perfectly balanced, so that large vibrations and noises are produced.
In the rotary fluid machine, a stroke from the end of the suction to the end of the discharge is 360.degree. in terms of an angle of rotation of a shaft, and therefore the problem that a pressure loss increases during the discharge stroke is less serious as compared with the reciprocating fluid machine. However, a fluid is discharged for each rotation of the shaft, and therefore a variation in a gas compression torque is relatively large, which invites vibration and noise problems as in the reciprocating fluid machine.
In the scroll fluid machine, a stroke from the end of the suction to the end of the discharge is as long as more than 360.degree. in terms of an angle of rotation of a shaft (usually, about 900.degree. in a scroll fluid machine put into practical use for air-conditioning purposes), and therefore a pressure loss during the discharge stroke is small, and, generally, a plurality of working chambers are formed, and therefore there is achieved an advantage that a variation in a gas compression torque is small, so that vibrations and noises are small. However, it is necessary to control a clearance between the volute wraps, engaged with each other, as well as a clearance between the end plate and the tip of the wrap, and therefore high-precision processing or working is needed, which invites a problem that the processing cost is high. Besides, since the stroke from the end of the suction to the end of the discharge is as long as more than 360.degree. in terms of the rotational angle of the shaft, the time for the compression stroke is long, which invites a problem that an internal leakage increases.
Proposed in Japanese Patent Unexamined Publication No. 55-23353 (Document 1) and U.S. Pat. No. 2,112,890 (Document 2) are a kind of displacement type fluid machines in which a displacer (hereinafter referred to also as "orbiting piston") for moving a working fluid does not rotate about its axis relative to a stationary member (hereinafter referred to also as "cylinder"), but revolves (that is, makes an orbital motion) relative to the stationary member with a generally constant radius, thereby conveying the working fluid.
In the displacement type fluid machine proposed in these publications, the piston having a radial configuration (in a plan view) formed by a plurality of arcs, and the cylinder having a radial inner surface (in a plan view) disposed a predetermined distance from the outer periphery of the piston are combined together, and the piston is caused to make an orbital motion within the cylinder, thereby conveying, compressing and expanding the working fluid.
The displacement type fluid machines disclosed in the above Document 1 and Document 2 do not have any reciprocating portion as in a reciprocating machine, and therefore the rotation shaft system can be completely balanced. Therefore, vibrations are small, and also the speed of relative slip between the piston and the cylinder is low, so that a friction loss can be reduced to a relatively small value, which is an essentially advantageous feature for the displacement type fluid machine.
However, the stroke from the end of the suction to the end of the discharge in each of the working chambers formed by a plurality of vanes (constituting the piston) and the cylinder is as short as about 180.degree. in terms of the angle .theta. of rotation of the shaft (this is about a half of that of the rotary type, and is about the same as that of the reciprocating type), and therefore the flow velocity of the fluid becomes high during the discharge stroke to cause an increase in pressure loss, which invites a problem that the performance is lowered. In the fluid machines disclosed in the above Document, a rotating moment, which is produced as a reaction force of the compressed working fluid and tends to rotate the orbiting piston, is exerted on the orbiting piston, and the arc-shaped portions of the orbiting piston bears such rotating moment. However, the compression working chambers formed by the orbiting piston and the cylinder during the stroke from the end of the suction to the end of the discharge are disposed in a concentrated manner on one side of the drive shaft, and therefore the rotating moment acting on the orbiting piston becomes excessive due to the pressure within the compression working chambers, and as a result the gap between the orbiting piston and the cylinder is enlarged to increase the leakage of the working fluid between the compression working chambers and the cylinder (because of temporary deformation and wear), and therefore there has been encountered a problem that the performance is lowered.
The rotating moment is born by the points of contact between the orbiting piston and the cylinder, and therefore there has been encountered a problem that the reliability is lowered.
The stroke from the end of the suction to the end of discharge in the compression working chambers formed by the orbiting piston and the cylinder is as short as about 180 degrees in terms of the angle of rotation of the shaft to increase the flow velocity during the discharge stroke and decrease a pressure loss, which invites a problem that the performance is lowered.